Circuits that interface with various types of sensors require analog front ends for amplifying and filtering sensor signals. To take advantage of increasingly dense technologies such as provided by metal oxide semiconductor (MOS) technologies, and the ability to integrate analog and digital circuits on the same silicon die, the analog front ends may be implemented in MOS technologies. Many times, the front end analog circuits use differential amplifiers to provide good noise rejection of sensor inputs or transmitted signals. Differential amplifiers designed in MOS technology for high performance applications will typically utilize high performance MOS devices implemented with thin gate oxide thickness. In these designs, a thin gate oxide thickness is typically employed to meet high performance requirements by providing fast signal response. As technology continues to scale to smaller and smaller dimensions, such as found in 90 nanometer (nm) technology, for example, the gate oxide layer also reduces in thickness in order to provide improved performance levels.
This reduction in oxide thickness is not without its problems. For example, with reduced gate oxide layer thickness to provide the required performance levels, there is an increase in gate leakage current. This increase is in part due to tunneling of charge carriers through the thin gate oxide and the consequent effect on gate leakage current and common mode voltage variations when operating throughout the input voltage range. The effects of gate leakage current can be considerable. For example, in a sample and hold circuit the voltage held in a hold capacitor connected to an input device will discharge at a rate, referred to as the droop rate, in part due to input gate leakage current of the input device. With an input device having a reduced gate oxide layer thickness, the gate leakage current could be relatively large resulting in a high droop rate.
For applications requiring high performance and high gain amplifiers in analog front end circuits, these problems must be addressed in order to meet the requirements in an existing process technology and to have the front end design be scalable to increasingly dense technologies.